invitations do wild things: you might even see "on the fifteenth day of October" -- it just depends on the form(ality) of the invitation. Go with what you like for the invitation; otherwise, follow the advice of E of G.
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Which of the below is correct for an invitation?
1. Monday, January 16, 2018, at 4:00 p.m.
2. Monday, January 16, 2018 at 4:00 p.m.